1. Field of the Invention
This invention relates to the protection of underground or immersed structures exposed to wet corrosion. In industrial practice many metal structures suffer from this technical problem, for example underground fuel storage tanks, off-shore structures, fluid transporting pipelines such as water, gas and oil pipelines, and other infrastructures for industrial, civil or military use.
The present invention is described with reference to underground pipelines, and involves one of its most difficult and interesting applications, but without in any way limiting it to this specific application.
2. Discussion of the Background
Liquid or gas transporting pipelines, such as gas, water or oil pipelines, are formed by welding together lengths of metal pipe, generally of steel, the continuous pipeline assembled in this manner being laid in its final seat, generally consisting of a sufficiently deep trench, and then covered in order to recreate the pre-existing environment and hence not hinder its further use. The assembled continuous pipeline is provided with protection against wet corrosion, as the environment in which the underground or immersed pipeline lies is very aggressive towards ferrous materials.
The integrity and preservation of the pipeline during its entire operating life must be ensured, both because of its high construction cost and, in particular, because fluid leakages must be prevented as they can cause danger, pollution or serious disturbance, in addition to representing an economical burden. The protection generally used consists of two different types of protection in mutual cooperation, namely "passive" protection consisting of a covering which isolates the pipeline from the surrounding environment, and "cathodic" protection which applies an electric potential for inhibiting possible electrochemical reactions which could attack the pipeline metal to the extent of forming a through hole.
The "passive" protection is generally formed by applying a continuous covering to the properly cleaned pipeline. This covering can be of two main types, the first type being coverings of hydrocarbon materials such as asphalts and bitumens which are generally applied hot in a number of layers together with fibrous material reinforcements, and the second type being synthetic polymer coverings such as polyethylene, polyvinylchloride and epoxy polymers, these being applied in the form of strips wound spirally about the pipeline and superimposing the side edges, or by die-casting. Web, felt or card made of glass wool, asbestos or other non-rotting fibrous material are used as protection and reinforcement. This protection is not on its own sufficient to protect an underground or water-immersed pipeline for the required time.
In this respect, the following should be noted:
no material is free of a certain porosity and permeability, even if perfectly applied, and hence a certain diffusion of the chemical species responsible for corrosive attack takes place through the protective layer, even if very slight; PA1 the sequence of operations involved in the preparation, covering, lifting, laying and burying of the pipeline can result in immediate slight damage or imperfections to the applied covering, these defects then triggering corrosion phenomena; PA1 the hydrocarbon or polymer materials and their reinforcements have a chemical and physical stability which is very high but is not absolute, particularly in relation to temperature or humidity changes; PA1 natural phenomena, such as earthquakes, landslips and floods, and accidental events can damage the pipeline passive protection. The "cathodic" protection protects the pipeline at those points in which porosity, damage or imperfect application of the covering have left the metal surface exposed to corrosive attack.
The variation in the state of the passive protection covering on the pipeline can be monitored by the method described in EP Patent Application Public. No. 0 411 689 in the name of the present applicant. Detection of local damage due to accidental events can be effected by the method described in EP application Public. No. 0 495 259.
According to these methods, the cathodic protection current applied to the pipeline is modulated by square waves. The application of square wave signals enables the resistive components and the capacitive components constituting the overall impedance of the pipeline-ground system to be separated and determined.
The present invention relates to the monitoring and location of any detachment of the protective covering from the metal walls of buried or immersed pipelines, due to various causes such as imperfect application of the covering, movement and deformation of the pipeline, ageing and brittling of the covering materials or applied adhesives exposed to an aggressive environment, or the development or seepage of gas or vapour between the pipeline and sheath. Such detachment is generally concentrated, and in contrast to normal covering decay it requires urgent local repair, otherwise it could cause rapid local corrosion to the extent of putting the entire pipeline out of use.
To highlight the characteristics of this problem, FIG. 1 shows a pipeline portion 1 with, towards the left, a defect in the covering 2 exposing a zone 3 of the metal wall, which is thus in contact with the ground 4. Towards the right there is a detachment of the covering 2 which creates a bubble between the wall and the covering, leaving a wall zone 5 exposed to corrosion. The pipeline is provided with a cathodic protection current generator 6 connected to the ground 4 via the earth plate 7. Numerous appendices 8 are distributed along the pipeline for the electrical connection.
At the defect in the zone 3 the cathodic protection current replaces the protection offered by the covering as there is electrical continuity between the ground 4 and the pipeline, the pipeline 1 therefore being still protected and not subject to corrosion.
In contrast, due to the effect of microporosity or of small gaps, over the long term there is migration into the bubble in the zone 5 of moisture, aggressive substances and aggressive microorganisms (sulphate-reducer bacteria colonies are particularly dangerous) which trigger concentrated corrosion on the metal wall of the zone 5.
Such corrosion can result in the development of gas phases which further extend the bubble and the corrosive attack zone. In contrast to the preceding case the metal wall does not have effective electrical continuity with the ground 4 because of the screening provided by the interposed raised covering 2, with the result that the cathodic protection current cannot effectively replace the covering in protecting the pipeline. It is therefore necessary to determine and locate early on those zones in which the covering has become detached from the pipeline in order to remedy this before such corrosion irreparably damages the metal walls of the pipeline.